Radio detection and ranging (radar) devices generally include a transmitter for illuminating an object with radio frequency energy "pulses" and a receiver for receiving energy reflected from the object. For instance, a single radio frequency pulse transmitted toward an object from the radar device will provide the distance from the radar transmitter to the object which reflected the energy. Resolution of images using conventional radar systems can be low, however.
Conventional synthetic aperture radar (SAR) systems are capable of providing a higher degree of resolution since SAR systems have a relatively large effective aperture size. More specifically, a synthetic aperture is created by putting a transmitter on a moving vehicle (e.g., airplane). As the vehicle moves a known distance at a constant velocity, the transmitter on board the vehicle transmits toward a designated object or area to be illuminated pulses of radio frequency energy over a period of time. In this regard, the resulting aperture effectively corresponds to the distance the vehicle moved during the period of time the transmitter was pulsing energy toward the designated object or area, which is significantly larger than the aperture of the typical radar systems.
Typically, for a conventional side looking SAR system mounted on board an aircraft, as the aircraft flies in the "along track" (i.e., azimuth) direction at a constant velocity, the radar antenna transmits a series of periodic radio frequency energy pulses to image a strip, and various stationary objects within the strip (e.g., on the ground) reflect radio frequency energy back to the SAR. As the radar antenna pulses periodically as it flies along in the azimuth direction, the distance between a given stationary radio frequency reflector or scatterer and the antenna/transmitter varies, whereby the distance is at a minimum at the point where the antenna/transmitter is broadside the stationary scatterer. Such distances can be processed to provide the location of the stationary object or objects reflecting or scattering the radio frequency energy within the strip being imaged.
In addition to distance, such conventional side looking SAR systems have also been used to produce images of the stationary objects (e.g., scatterers) reflecting or scattering the radio frequency energy within the strips. Initially, it should be noted that images from SAR systems are viewed from an "above the strip, downward looking" perspective, as opposed to a view from the flight path of the SAR system, which is off to a side of the imaged strip. Generally, two-dimensional images of illuminated strips have been produced. The two dimensions are the range (e.g., cross track) resolution and the azimuth (e.g., along track, perpendicular to range) resolution. As noted hereinabove, the range corresponds to the distance between the SAR and the stationary scatterer. The range resolution (e.g., degree of accuracy of stationary scatterer's position in the range direction) is typically dependent upon the width of the radio frequency pulses which are transmitted. Azimuth resolution (e.g., degree of accuracy of scattering object's position in the azimuth direction) is dependent upon the length of the synthetic aperture, which generally corresponds to the distance flown by the antenna of the SAR system. In particular, the azimuth position is dependent upon a scattering object's Doppler frequency of the reflected energy, which corresponds to the position of the SAR system relative to the scattering object. Processing of Doppler frequency shifts or offsets provides a location of scatterers in a scene being illuminated.
Of importance, for conventional side looking SAR systems, there is a fundamental assumption that all scatterers or targets in a strip or scene being illuminated remain stationary during the collection period so that the Doppler shift will behave properly to precisely locate the position of all scatterers in azimuth. If a scatterer is a moving object (e.g., an object with a constant or variable linear or nonlinear velocity, wind-blown trees or foliage, ships at harbor moving due to wave motion, etc.) within the scene or strip being illuminated, azimuth streaking can result as the Doppler shift incurs change due to the along track velocity of the moving scatterer. As such, there is a need to discern between azimuth streaking due to scatterers moving with a constant linear velocity, which are typically of primary interest, and azimuth streaking due to false movers or scatterers which are not moving at a constant linear velocity (e.g., clutter or background). Such discernment, however, has been especially problematic in instances where the reflected energy from the clutter or background, which can include wind-blown trees, foliage or the like, is greater than the reflected energy from the scatterers moving at a constant linear velocity (e.g., low signal-to-background scatterers), which are typically the targets of primary interest. In such instances, azimuth streaking from such low signal-to-background scatterers moving at a constant linear velocity has not been detectable. This is especially problematic for space-based SAR systems, which are typically single aperture systems, rather than multi-aperture systems, due to weight and power constraints.